1. Field of the Invention
The invention is related to a pixel circuit for liquid crystal display, wherein a digital circuit is installed at a pixel of the liquid crystal display for processing static image. The digital circuit works with an analogue circuit for lowering the power consumption so as to accomplish power saving function of a pixel circuit for liquid crystal display.
2. Description of the Prior Art
Liquid crystal display (LCD) is widely used in notebook computers and various apparatus with display functions. An image pixel driving circuit used in the LCD is an analogue circuit. Among prior art LCD elements, passive or active matrix liquid crystals such as thin film transistor (TFT) and twisted nematic (TN) are used. A schematic view of exemplary circuit of a prior art pixel circuit is shown in the FIG. 1. The circuit shown in the FIG. 1 is used as a basic unit to form a LCD. All unit circuits share a scanning line 103 and data line 105. FIG. 1 shows a circuit of active matrix TFT LCD 101. The architecture of image pixel is composed of TFT LCD 101, a capacitor 107 a liquid crystal unit 109. An analogue voltage is required to write into the capacitor 107 so as to display gray level image, and a scanning line 103 as the circuit switch. When a signal from scanning line 103 indicates to switch the liquid crystal unit on, the data line 105 then charges/discharges the capacitor 107. Due to the malfunction of TFT 101, a current leakage may occur and result in gray level loss. To prevent aforementioned phenomenon and render a good gray level display, the data line 105 is required to continually charge/discharge TFT 101. Said operation results in a refresh rate data, which serves as an important reference for LCD performance.
In the prior art, a surface stabilized ferroelectric liquid crystal (SSFLC) is also used to form a LCD. The SSFLC has spontaneous polarization. When an external electric field is applied, the direction of the spontaneous polarization reverses and such direction is then retained. As a result, when the LCD displays static image, it's no longer required to continually writing signals into pixels, neither is required to continually charge/discharge data line, so as to reduce power consumption. The drawback of the method is that such display only shows black and white. A gray level display requires complicated circuits such as pulse width modulation (PWM).
In order to resolve the aforementioned drawbacks of pixel circuit for liquid crystal display such as high power consumption or requirements to use complicated circuits, a digital circuit is employed at a pixel of the LCD in the present invention, such frequent display refresh is eliminated and the power consumption is reduced.